The Roy Hill mine in the Pilbara.
A whistleblower at Gina Rinehart's Roy Hill iron ore project in Western Australia's Pilbara region claims workers on 457 visas are working excessive hours and being grossly underpaid.
The CFMEU says it has asked Employment Minister Eric Abetz to start an urgent investigation.
The whistleblower says up to 200 white-collar 457 visa workers, about half of whom are Korean nationals aged under 30, are clocking up more than 84 hours a week. Many are female.
They are employed by the contractor Samsung C&T and being paid about $16 an hour, the union says.
Many are not working in the occupations approved for their visas - a breach of the sponsoring employer's obligations, the CFMEU claims.
CFMEU national secretary Michael O'Connor said the alleged exploitation was a very serious concern.
"The allegations come as Roy Hill chief executive Barry Fitzgerald revealed that 457 visa workers were no longer required on the project because plenty of Australian workers were now available due to the downturn in construction," Mr O'Connor said.
"In the Australian Mining publication last month, Mr Fitzgerald said `more than 5500 people had applied to work at Roy Hill since December, with some positions attracting more than 600 applicants'."
The company, which was the first to sign an enterprise migration agreement with the federal government in May 2012, was being sought for comment.